Post #10046

Screw Threads. There are many profiles of screw threads but the most common is 60degree. What's magical about 60degree thread angle is that the cos of 60 degrees is 0.5. So the depth of a thread is 0.5 on one side , or 1.0 of the diameter. This means that you can calculate the inner diameter of a hole you intend to thread by taking the 1/(number of threads per inch) and subtracting that from the nominal diameter of the bolt. Here's an example... 1/4-20 thread... major diameter is 1/4inch (0.25"), the threads take up 1/20" (0.05). So 0.25-0.05=0.20. So the pilot hole (minor diameter) should be 0.20inches. Metric is even easier... M6x1 means the major diameter is 6mm and the thread pitch is 1mm, so 6mm-1mm= 5mm. The pilot drill should be 5mm. There are variables that machinists apply to change the fitment. Depending on the application you may want a small gap between the bolt threads and nut threads for dirt or grime, or you may want a tight fit. But this is good enough.